Among the various kinds of dc voltage converters that have become known is a common circuit in which the input dc voltage is chopped or is caused to produce periodic relaxation oscillations. The signals thus are transformed to a desired voltage range by means of a transformer with a suitable transformation ratio and then rectified.
Regulation circuits are also known in which the output voltage of dc voltage converters are regulated to a constant value, in order to counteract changes of the output voltage in the case of variations of the input voltage or of the output loading. Such regulation is carried out according to the known voltage regulator principles, so that, for example, the dc converter input voltage is compared with a reference voltage and the voltage difference is utilized to exert a control for regulating the dc converter input (primary side regulation). The dc converter output voltage can be correspondingly regulated. The disadvantage of this procedure is that large amounts of energy are used up in the regulating process itself. This is all the more disadvantageous because dc converters frequently serve for generating higher or more stabilized dc voltages from the voltage of a battery, for example the battery of a motor vehicle electrical system, for instance, where the energy consumption is an important consideration in the performance of any electrical function.
In the publication BBC-Nachrichten, 1974, Issue 10, p. 425, a circuit was disclosed that reduced the above-described disadvantages by using a keyed type of regulation. In this case the difference between the actual voltage and the desired voltage at the output of a dc converter was used for control of the keying ratio of the chopper. By interposition of a suitable LC circuit, the power output was indicated. Compared to a circuit with continuous regulation, this one has the advantage that the switching losses of a switching transistor are substantially smaller than the losses of a transistor acting as a variable valve, as used in a continuous control. There is the disadvantage, however, of the relatively high expense for the switching circuits.
In U.S. Pat. No. 3,263,124, a circuit is disclosed in which a Wien bridge oscillator is used instead of a chopper. The output is fed back to the base of a transistor on which there is applied in normal operation a desired or comparison voltage, through a Zener diode. If the output dc voltage increases to a certain value, the transistor is put in its conducting condition and short-circuits the coil of the Wien bridge oscillator. The disadvantage of this arrangement is that the frequency of the oscillator varies strongly over the regulation range. This is reflected on the one hand in transformer losses, and on the other hand the frequency can get into the audibility range of the human ear and become noticeable as a disturbing whistle.